Alkaline granitic magmatism of the Western Transbaikalia: Petrogenetic and geodynamic implications from U-Pb isotopic–geochronological data

Abstract

The Mongolian–Transbaikalian volcanoplutonic belt extends for almost 2000 km through northern Mongolia and Transbaikalia and hosts several hundreds of plutons of alkali feldspar syenites (AFS) and peralkaline granites (PA) that formed at 280–270 and 230–210 Ma. The giant (~220 km2 in area) Sherbakhta pluton in the Vitim plateau (western Transbaikalia) is located and composed of metaluminous (A/CNK ≤1), high ferroan AFS syenites (10.7–12.3 wt% Na2O + K2O; NK/A < 1) and A-type peralkaline granites (8.7–9.5 wt% Na2O + K2O; NK/A 0.95–1.05). The U-Pb (SHRIMP-RG, LA-ICP-MS) age of the pluton is 260 Ma. The isotope and geochemical characteristics of the studied pluton (ɛNd(T) –2.4 to −2.9; TNd(DM-2) 1.25–1.27 Ga; ɛHf(T) 1.1–0.6, THf(DM-2) 1.02–1.05 Ga, δ18О of quartz 7.3–8.22‰) indicate that the parental trachytic–trachyandesitic melt is compositionally similar to high-Sr syenites of the Sherbakhta pluton and derived by the melting of the mafic, moderately potassium-rich lower crustal source. The magmatic evolution of the parental melt toward a residual ferroan peralkaline granitic melt in the intermediate chamber was controlled by fractional crystallization of sodic plagioclase and K-feldspar, followed by amphibole and aegirine under low oxygen fugacity. The A-type granitoids in the Transbaikalia region formed in the intracontinental geodynamic setting, probably as a result of impact of the mantle plume on heated plastic crust of a young orogen, which formed during the Late Hercynian fold thrust deformations.